NEWS FEATURE

BY LEE BILLINGS

om Driscoll would be happy if he

never heard the phrase Harry Potterstyle invisibility cloak again. But heknows he will. The media cant seemto resist using it when they reportthe latest advances in metamaterials arrays of minuscule elements that bend, scatter, transmit or otherwise shape electromagneticradiation in ways that no natural material can. It istrue that metamaterials could, in principle, routelight around objects and render them invisible, notunlike the cloak of a certain fictional wizard. Andmany metamaterials researchers are trying to makecloaking a reality, not least because the military haseagerly funded the development of such capabilities.However, if such applications ever come to passit will be decades from now. Technologies closer to

commercialization are of more interest to Driscoll, a

physicist who oversees metamaterials commercialization at Intellectual Ventures, a patent-aggregation firmin Bellevue, Washington. Applications such as cheapersatellite communications, thinner smartphones andultrafast optical data processing are where metamaterials are poised to make a huge impact, he says.Researchers still face some daunting challenges,he adds notably, finding cheap ways to fabricateand manipulate metamaterial elements on a scaleof nanometres. But the first metamaterial-basedproducts are expected to come onto the market in ayear or so. And, not long after that, Driscoll expectsthat average consumers will start to enjoy the benefits, such as faster, cheaper Internet connectivity onboard planes and from mobile phones. Such applications, he says, will move from being the stuff of

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REF. 3

Engineered structures with bizarre optical

properties are set to migrate out of thelaboratory and into the marketplace.

FEATURE NEWSpeoples fantasies to becoming things theycant contemplate living without.The first laboratory demonstration of a metamaterial was announced in 2000 by physicistDavid Smith and his colleagues at the Universityof California, San Diego1. Following up on theoretical work done in the 1990s by John Pendryof Imperial College London, these researchersshowed that an array of tiny copper wires andrings had a negative refractive index for microwaves meaning that microwave radiationflowing into the material is deflected in a direction opposite to that normally observed (seeWave engineering). That triggered intenseinterest in metamaterials, in part because theability to bend radiation in such a way hadpotential for creating invisibility cloaks.Since then, Smith and others have exploreda host of variations on the metamaterial idea,often looking to manipulate radiation in waysthat have nothing to do with a negative refractive index. They have also moved beyond staticarrays, devising techniques to change the waythe elements are arranged, how they are shapedand how they respond to radiation. The resulting materials can do things such as turn fromopaque to transparent or from red to blue allat the flick of a switch.

Market movers

In January, Smith, now at Duke University in

Durham, North Carolina, took on a concurrent role as director of metamaterials commercialization efforts at Intellectual Ventures.I felt that the time was right, and we didntneed to do any more science for some of thesethings, he says.A test case may come as early as next year.Kymeta of Redmond, Washington, a spin-offfrom Intellectual Ventures, hopes to market acompact antenna that would be one of the firstconsumer-oriented products based on metamaterials. The relatively inexpensive devicewould carry broadband satellite communications to and from planes, trains, ships, carsand any other platform required to functionin remote locations far from mobile networks.At the heart of the antenna the details ofwhich are confidential is a flat circuit boardcontaining thousands of electronic metamaterial elements, each of which can have itsproperties changed in an instant by the devicesinternal software. This allows the antenna totrack a satellite across the sky without havingto maintain a specific orientation towards it,the way a standard dish antenna does. Instead,the antenna remains still while the softwareconstantly adjusts the electrical properties ofeach individual metamaterial element. Whenthis is done correctly, waves emitted fromthe elements will reinforce one another andpropagate skywards only in the direction of thesatellite; waves emitted in any other directionwill cancel one another out and go nowhere.At the same time and for much the samereason the array will most readily pick up

signals if they are coming from the satellite.

This technology is more compact thanalternatives such as dish antennas, says Smith.It offers significant savings in terms of cost,weight and power draw. Kymeta has alreadyperformed demonstrations of this technologyfor investors and potential development partners. But Smith cautions that the company hasyet to set a price for the antenna and that itmust still work to bring production costs downwhile maintaining the strict performancestandards that regulatory agencies demandfor any device communicating with satellites.Kymeta has shared so few details of itsantenna that researchers say it is hard to offeran evaluation. But Smith is highly regardedin the field. If Kymeta brings the product tomarket, it may first offer its antenna for useon private jets and passenger planes. If buyersrespond well, the company hopes to incorporate the technology into other product lines,such as portable, energy-efficient satellitecommunication units for rescue workers orresearchers in the field.In January, Smiths group turned heads whenit announced its demonstration of anothermetamaterial device: a camera that can createcompressed microwave images without a lensor any moving parts2. One important application of the device might be to reduce the costand complexity of airport security scanners.In their current form, these scanners have tophysically sweep a microwave sensor over andaround the subject. This produces an unwieldyamount of data that has to be stored before itis processed into an image. The Duke groupsdevice requires very little data storage. It takesnumerous snapshots by sending beams ofmicrowaves of multiple wavelengths across thetarget at about ten times per second. When themicrowaves are reflected back by the subject,they fall on a thin strip of square copper metamaterial elements, each of which can be tunedto block or let through reflected radiation. Theresulting pattern of opaque and transparent elements can be varied very rapidly, with each configuration transmitting a simplified snapshot ofa scanned object into a single sensor. The sensormeasures the total intensity of radiation fromeach snapshot, then outputs a stream of numbers that can be digitally processed to reconstruct a highly compressed image of the subject.This is admittedly just a first step: demonstrations carried out so far have been crudeaffairs restricted to two-dimensional imagesof simple metallic objects. Expanding it tothree-dimensional images of complex objectsremains a challenge. But if that challenge canbe overcome, says Driscoll, airports could retirethe bulky, expensive, slow booths that currentlyconstitute security checkpoints, and instead usea larger number of thin, inexpensive metamaterial cameras hooked up to computers. Such ashift, Driscoll says, could extend security scanning to rooms, hallways, and corridors throughout airports and other sensitive facilities.

WaveengineeringMetamaterial elements scatter incomingradiation in very precise ways. They can be anyshape; common examples include spheres, rings,crosses and chevrons. Their electromagneticproperties can often be changed by software.

The spacingbetween theelements can vary,but is always lessthan the wavelengthof the radiation.

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NEWS FEATUREIn the meantime, a key research goal forSmith and his group is the development ofrobust and marketable metamaterial devicesthat are not restricted to radio, microwaveor infrared wavelengths. If the technologiescould be made to work with visible light, theywould become much more useful for applications such as fibre-optic communications orconsumer-oriented cameras and displays.It wont be easy, cautions StephaneLarouche, a member of Smiths research teamat Duke. For any given type of radiation, heexplains, metamaterials can wield their exoticpowers only if the elements are smaller andmore closely spaced than the wavelength ofthat radiation. So the shorter and shorter thewavelength we wish to use, the smaller eachmetamaterial element must be, says Larouche.In the microwave and radio regions of thespectrum, this is relatively easy: wavelengthsare measured in centimetres to metres. But anoptical metamaterials elements would have tomeasure considerably less than a micrometre.That is not impossible: todays highperformance microchips containfeatures only a few tens of nanometres across. But unlike those essentially static features, says Larouche,the metamaterial elements in manyapplications would need to incorporate ways for software to change their properties dynamically as needed. Too often we havegorgeous ideas, he says, but we have no wayof fabricating them.

of gold metamaterial elements carved out of a

60-nanometre-thick silicon wafer using electron-beam lithography techniques developedfor the microchip industry. The elements arefixed, so cannot be tuned after fabrication. Butby selecting a specific size and spacing duringthe manufacturing process, physicists can shapelight of a chosen wavelength in exactly the rightway to make it come to a point.Capasso warns that commercial applicationsof such flat lenses are probably still a decadeaway. This is partly because silicon is a rigidand fragile substrate for etching the elements;researchers are looking at more robust andflexible alternatives that would be easier tohandle on the production line. They are alsolooking for better ways to control the carvingof the nanoscale elements, which has to bedone very precisely.But once the technology is mastered, saysCapasso, one obvious application is in smartphone cameras. Lenses, along with batteries,are among the most stubborn limiting factors

The team has since been working to refine the

superlens concept; in 2007 the researchersadvanced the idea by developing hyperlensesfrom curved, nested layers of compounds suchas silver, aluminium and quartz6. The lenses notonly capture evanescent waves, but can also feedthem into a conventional optical system. Ultimately, this could allow sub-wavelength detailsto be viewed through the eyepiece of a standard microscope. But the complex structure andbehaviour of hyperlenses makes them difficultto manufacture and use in this way.

Reversible focus

By pairing conventional optics with superlenses and hyperlenses based on metamaterials, Zhang hopes eventually to findapplications far beyond microscopy. Just asthese constructs can magnify sub-wavelengthdetail, they can also be run in reverse, directing beams of light into sub-wavelength focalpoints a property of potentially revolutionary importance for fabricating minusculestructures using photolithography.If superlenses and hyperlenses canbe harnessed for this purpose, theultra-fine beams of light could beused to etch much smaller featuresthan is possible today. This couldgreatly increase the density of datastorage on optical drives, as well as the number of components that can be crammed ontocomputer chips.Smith is cautious on that score, pointing outthat hyperlenses and superlenses tend to dissipate substantially more of the light energypassing through them than other advancedlithographic techniques now in development.This, he says, makes them prime examples ofstrong and compelling science that is not yetpractical for any sort of product path at opticalwavelengths. But, he adds, Zhangs efforts areheroic experiments that illustrate the potential of metamaterials in a fundamental way.Zhang concedes that hyperlenses and superlenses are not yet ready for prime time, butbelieves there is plenty of room for ongoingresearch to change that situation in the comingyears. The economic impact could be huge, hesays. I am cautiously optimistic that metamaterials, superlenses and lithography will provetruly revolutionary. If people arent too shortsighted, what we can do with metamaterials willbe limited only by our imaginations.

Metamaterials arepoised to make a hugeimpact.

Flat focus

Despite these difficulties, workable designs for

optical metamaterials have begun to emerge.One was published in March3 by a groupworking under Nikolay Zheludev, a physicistat the University of Southampton, UK, whodirects a research centre focused on metamaterials at Nanyang Technological Universityin Singapore. The teams device can greatlyalter its ability to transmit or reflect opticalwavelengths by means of nanometre-scale,electrically controlled metamaterial elementsetched from gold film; it could one day serveas a switch in high-speed fibre-optic communications networks.Meanwhile, because it is so hard to make andcontrol three-dimensional metamaterial arraysat optical scales, some researchers are focusingon two-dimensional metasurfaces. In August2012, a group led by Federico Capasso at Harvard University in Cambridge, Massachusetts,unveiled a flat metamaterial lens that can focusinfrared light to a point in much the same wayas a glass lens4. I dont want to claim absolutenovelty in this, Capasso says, but I believe weare the first group to so clearly put flat optics onthe agenda for commercial applications.A conventional lens relies on refractionto bend light to a point by passing it throughvarying thicknesses of glass. Capassos lenspasses light through a two-dimensional array

in smartphone thickness, he says, speculating

that a smartphone incorporating a flat cameralens could potentially be made as thin as acredit card. The flat lens also avoids aberrations that plague glass lenses, such as the coloured fringes created by the inability to focusall wavelengths to the same point. This meansthat Capassos flat lens could also be used tomake better, aberration-free microscopes.As good as they might ultimately be, the flatlenses would still be subject to the diffractionlimit, which dictates that no conventional lenscan resolve details much smaller than the wavelength of the light that illuminates its target.This limit averages about 200 nanometres forvisible light. But metamaterials offer a means offabricating superlenses that could surpass suchlimits, allowing researchers to see sub-wavelength details of target objects such as virusesor the ever-changing structures in living cells.The key is to recognize that the missingdetails are still there, carried in evanescentwaves of reflected light that die away very rapidly with distance from the illuminated object.Normally, these waves have effectively vanishedbefore they can be captured and focused by alens. But a metamaterial superlens designedto be placed within tens of nanometres of anobject can pick up and magnify these waves.An early proof-of-concept superlens wasdemonstrated in 2005 by a group workingunder Xiang Zhang, a physicist at the University of California, Berkeley5. Zhangs groupproduced a simple metamaterial consisting of a35-nanometre-thick layer of silver in a sandwichwith nanoscale layers of chromium and plastic.